One day you can work 5 hours and end up exhausted solving something. To keep sitting in your chair wasting time without being efficient or doing tasks that do not contribute much and that you are not going to do them in the best possible way is not worth it. The important thing is not the hours you work, but being efficient and making the most of your time.

There are days that you can work less hours and other days that you work more hours until you solve something. If you are a responsible person, they have to trust you. You work for objectives, not to be warming the chair a number of hours. In jobs like this a clear mind is infinitely more effective than a tired mind.

This is something you are not going to do every day, it is not a vacation or a waste of time. It's the opposite, it's about being efficient. A complex task if you are tired can take you much longer than if you have a clear mind and not only that, but you can make mistakes that you will pay in the future.

Doing mechanical tasks is not the same as designing an architecture or creating a highly complex function.